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Electric boiler wiring

KMA1987

Hoping someone can help me with a question on the wiring requirements for a 12kw electric boiler.

For context, I’m attempting to have a botched installation rectified after the manufacturer said the warranty had been invalidated by an incorrect installation (both plumbing and wiring aspects). I’m not an electrician and I’ve had a couple of electricians look at it now and received different answers.

The boiler in question is a Trianco Aztec Classic 12kw (installation manual here: https://heatingpartswarehouse.co.uk/wp-content/uploads/2023/06/Aztec-Classic.pdf). The relevant page for the wiring instructions is 16.

My specific question is whether the boiler and the external wiring controls (we have a Reliance 8 zone wiring centre) should be on separate circuits? The wiring requirements state: “Miniature circuit breakers (MCB) MUST be fitted between RCD unit and boiler and RCD and any external controls….An additional MCB rated 6A will be required to supply the external controls.”

The current setup is the boiler and controls are on a 63A type B RCBO, with a switched fused spur on the same circuit for the controls. Whether this was ever correct/appropriate for the previous boiler I can’t say, but the installer has obviously just tried reuse what was already there on a like-for-like basis with the new boiler (different manufacturer/model).

One electrician has said the external controls should have been installed on a separate circuit with its own MCB, and the other that it can be on the same circuit and just replace the fused spur with a 6A MCB.

Happy to provide more information and grateful if there’s anyone with experience with electric boilers that may be able to help.

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  • 0

    Having wired some of these boilers I know there’s more than one instruction book and the issues arise due to being able to carry out electrical safe isolation when there are two electrical circuits and MCBs supplying a single appliance.

    The instructions are convoluted, but basically what it is says is if you put an enclosure next to the boiler with an RCD main switch and the two MCBs for the boiler and controls, the RCD can be used for safe isolation as there is not electrical separation between the two circuits as they can  share a neutral, which also means you can’t use two RCBOs.


    electricaldealsdirect.co.uk/.../classic_man.pdf

  • 0 in reply to Sparkingchip

    Quote “The current setup is the boiler and controls are on a 63A type B RCBO, with a switched fused spur on the same circuit for the controls.”

    If the 63-amp RCBO is the isolator, is it next to the boiler?

    As seen in the instructions above, the installation instructions state “Note: RCD UNIT can be used as the isolating switch if mounted close enough to the boiler.”.

    If the boiler is not wired correctly the service engineer could be electrocuted if they are not very careful when carrying out the safe isolation and there are two electrical circuits running into the same appliance.

  • 0 in reply to Sparkingchip
    Sparkingchip said:
    if you put an enclosure next to the boiler with an RCD main switch and the two MCBs for the boiler and controls

    Ah yes, a consumer unit.

    Sparkingchip said:
    If the boiler is not wired correctly the service engineer could be electrocuted if they are not very careful when carrying out the safe isolation and there are two electrical circuits running into the same appliance.

    A very good point. A 4-pole switch could be used as a means of isolation.

  • 0 in reply to Sparkingchip

    Thanks for your reply.

    I know not ideal but if helpful here's some photos of the current setup - there's 1.) a 4 pole rotary isolator switch, 2.) a 3A switched fused spur (labelled "boiler control isolator") and 3.) another isolator switch at the bottom labelled "UFHeating isolator". Why there is both 2.) and 3.) I'm not sure - they both seem to isolate the UFH wiring centre and room thermostats.(You can ignore the switches/timer in the middle as they're for the separate hot water cylinder next to the boiler). All of these are on the 63A RCBO at the consumer unit labelled "Boiler supply". 

  • 0 in reply to KMA1987

    To be safe the control circuit switched fused connection unit must be connected to the outgoing terminals of the boiler rotary isolator, not the input terminals, so that the rotary isolator fully isolates both live feeds into the boiler.

    There is no way of knowing if its wired correctly from the photos, but even if it is safe it doesn’t comply with the manufacturers installation instructions.

    Really the rotary isolator should isolate both circuits and have a label on it saying “Boiler and controls isolator”, then  boiler controls should be fused down through an unswitched fused connection unit to make service engineers use the rotary isolator.

  • 0 in reply to KMA1987
    KMA1987 said:
    there's 1.) a 4 pole rotary isolator switch, 2.) a 3A switched fused spur (labelled "boiler control isolator") and 3.) another isolator switch at the bottom labelled "UFHeating isolator". Why there is both 2.) and 3.) I'm not sure - they both seem to isolate the UFH wiring centre and room thermostats.

    It would make sense if the rotary switch isolates everything, and the FCU and other switch isolate the controls and heating element separately.

    So, if you just open the lowest switch, does the display remain on please?

  • 0 in reply to Sparkingchip
    Sparkingchip said:
    To be safe the control circuit switched fused connection unit must be connected to the outgoing terminals of the boiler rotary isolator, not the input terminals, so that the rotary isolator fully isolates both live feeds into the boiler.

    Isn't that why it has 4 poles?

  • 0 in reply to Sparkingchip

    Both electricians who have inspected the installation have both said the rotary isolator hasn’t been wired correctly as it is not isolating neutral.

  • 0 in reply to KMA1987

    Neither, I suspect, does the RCBO. If I am correct, that is another departure from the manufacturer's instructions, but I dount that it presents a hazard.

  • 0 in reply to Chris Pearson

    The only thing that switches off the boiler display is the rotary isolator. Both the “boiler control isolator” and “UFH isolator” turn off both the UFH wiring centre and room thermostats; they don’t do anything to the boiler or its display. 

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  • 0 in reply to Chris Pearson

    The only thing that switches off the boiler display is the rotary isolator. Both the “boiler control isolator” and “UFH isolator” turn off both the UFH wiring centre and room thermostats; they don’t do anything to the boiler or its display. 

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  • 0 in reply to KMA1987
    KMA1987 said:
    The only thing that switches off the boiler display is the rotary isolator. Both the “boiler control isolator” and “UFH isolator” turn off both the UFH wiring centre and room thermostats; they don’t do anything to the boiler or its display. 

    I see. The boiler display is part of the boiler.

    The UFH wiring centre and the thermostats are what the manual calls, "external controls". I cannot see any real reason why the various bits should not be isolated independently. Neither BS 7671, nor the manufacturer's instructions (as I read them) requires isolation of the neutral. If it is necessary, the main switch may be opened.

    I think that the situation could be improved, but I do not think that is necessary. A C3 rather than a C2 in the electrical world.

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